Renal ischemia is a major cause of acute renal failure. Ulinastatine (UTI), a protease inhibitor, has been reported to help protect against ischemia and various types of shock; however, its mechanism for these protective effects is not fully known. We have investigated the effects of UTI on renal ischemia and reperfusion by measuring the tissue pO2 simultaneously in the renal cortex and medulla in rats. The principal experimental protocol was 30 min renal ischemia (by clamping the abdominal aorta) followed by 60 min reperfusion. Treatments were 0.9%NaCl 1ml/kg (NS group) or UTI 50000U/1ml/kg (UTI group) administered 30 min before ischemia. In the NS group, the control pO2 values of renal cortex and medulla were 11.7 +/- 2.3 and 14.4 +/- 5.7 mmHg respectively, whereas those in the UTI group were 18.9 +/- 4.0 and 24.5 +/- 4.2 mmHg. The pO2 of the cortex and medulla decreased to less than 2 mmHg during ischemia in both groups. At 60 min after reperfusion, the pO2 values in NS group were not fully restored (7.6 +/- 2.3 and 6.9 +/-1.4 mmHg), whereas those in the UTI group were completely restored to the control value (21.1 +/-4.8 and 25.5 +/- 2.1 mmHg) . These data indicate that administration of UTI before ischemia increases the renal cortical and medullary pO2 after reperfusion.